motory requirements on a variety of substrates, radiating from at least two proto-molluscan body plans, one with a simple blood vascular system and the other with a hemocoel. Once a successful new locomotory system was evolved on one substrate type, invasion and reinvasion of other substrates became possible and the diversity of types increased further. These worms have left us an array of trace fossils and evidently little else, except their descendants, among whom we number.
The more complex of the late Precambrian worms must have been higher invertebrates in every sense, with, by invertebrate standards, sophisticated organ systems consisting of appropriately specialized tissues and these in turn composed of differentiated cell phenotypes that, judging by the body plans of living organisms, probably numbered in the 40s. Other worms were clearly less complex. For those worms that gave rise to Phanerozoic clades, it is expected that their primitive members were only minimally more complex than their ancestors, and it is plausible that the ancestral body plans were precursory to their Phanerozoic descendants. These considerations, and the evidence reviewed above, imply that many of the branching events required of metazoan diversifications occurred well before the onset of the Cambrian explosion. After the branchings, complexity continued to increase within many clades. For example, proto-mollusks gave rise to several distinctive hemocoelic and coelomic body plans, and early mollusks gave rise to the relatively complex cephalopods. The pattern of branching during the Vendian appears as a more-or-less orderly accumulation of a series of derived features. This has been the usual interpretation of the metazoan diversification pattern, more-or-less faute de mieux, though occasionally it has been challenged by proponents of a bush or grass-like pattern of many parallel lineages rising from a common ancestral clade (Nursall, 1962), which in the latest version is composed of flatworms (Willmer, 1990; Barnes et al., 1993). It appears more likely that the body plans from which numbers of phyla evolved were proto-molluscan.
Internal Factors. The mode of evolution that might account for the observed explosive appearance of body plans in the fossil record must have involved changes in genetic regulation within many lineages, including repatternings of gene expression and respecifications of cell fates and movements, but perhaps with little increase in levels of cell differentiation. Evidence of the nature of the transcriptional regulation of these processes is beginning to accumulate. It is clearly of interest to study the similarities and differences among the regulators that mediate